Polymer and use thereof in the production of paper and board

ABSTRACT

As additive for the production of paper and board to improve the strength of a wet web a copolymer stabilized with a cationic polysaccharide is proposed in which copolymer an acrylamide of metacrylamide, vinylic carboxyl acid and optionally other vinylic monomers are used as monomers. The wet strength of a web means the strength of a paper or board web during production at a dry solids content of 20-60%. The additive according to the invention improves, in addition to the wet strength of the web, also the surface resistance of the product. Compared to the additives of the prior art, a small relative ratio of the number of the side chains to the cationic groups is used, because a polysaccharide having a higher cationic character has been found to provide better stability.

FIELD OF THE INVENTION

[0001] The invention relates to the chemistry of additives for paper making. In particular, the invention relates to a polymer stabilized with a polysaccharide and being generally used as an additive in the production of paper.

BACKGROUND OF THE INVENTION

[0002] Various paper making additives are used to improve the properties of paper. Water soluble additives such as polyamides, polyamide-epichlorohydrin resins, melamin resins, urea-formaldehyde resins, polyacrylamides as well as preferably starch together with its derivatives are used to improve the strength properties. Starch is an advantageous and preferred raw material of natural origin. The amount of starch required to improve the strength properties is relatively large.

[0003] As wood fiber is anionic in character, native starch has a quite low affinity thereto. Therefore, starch has been chemically modified into the cationic direction. Starch is cationized by attaching substituents having an ionic nitrogen group, such as primary, secondary, tertiary or quaternary ammonium groups, to the carbohydrate backbone. As a measure of the cationic character, the relative ratio of substituted to unsubstituted glucose units, i.e. a degree of substitution, is used. The cationic character can also be expressed as a percentage amount of ionic nitrogen based on the weight of the substituted starch.

[0004] Of the strength properties of paper, wet and dry strength properties are to be viewed separately. The dry strength properties are usually improved by adding hydrogen bonds to OH groups of the fiber. The paper strengthening additives of this kind include hydrophilic, water-soluble polymers such as starch, carboxymethylcellulose or synthetic polymers. These include polyacrylamides of which anionic, cationic and amphoteric are used. Their use is limited by a relative high price.

[0005] By the term “wet strength” is usually meant the strength of a finished paper when it becomes wet, which strength is obtained by the formation of covalent bonds between the fibers. This property is of importance when making paper e.g. for filter paper or kitchen paper, and it can be be obtained through reactions between fibers and additives at a high temperature of the drying section of a paper machine. Another wet strength, the strength of a wet web during the production of paper is of great importance for the runnability of the machine, especially when proceeding to a drying section, because the web has been fully supported up to that point. It has been tried to improve this property mainly by increasing the dry solids content of the web in the press section.

[0006] In order to combine the properties of polyacrylamides and starches to improve the wet strength of paper, products have been prepared in which acrylamide or metacrylamide and an unsaturated organic acid are polymerized in the presence of starch. In this manner starch is formed to which polymerized side chains have been bound. The starch is preferably cationic, having typically a degree of substitution of 0.01-0.70.

[0007] Japanese patent application JP 2-26944 discloses a polymer of an acrylamide or metacrylamide and methacrylamide acid, prepared in an aqueous solution containing a cationic polysaccharide. In addition to the tensile strength, attempts to improve the tear strength has been made. The cationic character of the polysaccharide is low, and the nitrogen content is 0.4%. In an embodiment, a product is prepared by polymnerizing 50-95 mol-% of (meth)acrylamide and 5-50 mol-% of an α, β-unsaturated carboxylic acid in an aqueous solution containing 40-90 parts per weight of a cationic polysaccharide. In a further embodiment, a vinylic component to be polymerized is entirely in the form of amide, and after the polymerization, part of the amide groups is hydrolyzed to carboxylic groups.

[0008] The goal of Japanese patent application JP 3-213597 is to reduce the bending fracture of paper and paperboard. Said publication describes an amphoteric additive for paper making, which has been prepared by polymerizing 30-96 mol-% of methacrylamide or acrylamide and 4-50 mol-% of an α, β-unsaturated carboxylic acid or its salt in the presence of cationic starch, the aqueous solution containing 20-90 parts per weight of cationic starch. The effect has been reported to be due to the advantageous relative ratio of cationic groups of the starch to the anionic groups of the carboxylic acid, because of which the product is strongly adsorbed on pulp.

[0009] In the patent applications described above, the number of the formed side chains of the starch is large in relation to the number of the cationic groups, because low cationic starch was used.

[0010] Thus additives are used in the production of paper to improve the complex of fiber and a binder. The runnability of paper in a machine can be improved by web wet strength improving agents. The present invention relates particularly to the wet strength of a web, i.e. to the strength of a paper or paperboard web during the production with a dry solids content of about 20-60%.

DISCLOSURE OF THE INVENTION

[0011] After intensive researches we have found a copolymer stabilized by a cationic polysaccharide to be an advantageous additive in the production of paper and paperboard, in which copolymer acrylamide or metacrylamide, vinylic carboxylic acid and optionally other vinylic monomers, e.g. alkylaminoacrylate or metacrylate, are used as monomers.

[0012] We have found that for the strength properties, a much smaller ratio of the numbers of side chains to the cationic groups is advantageous compared to the above described prior art, because a polysaccharide having a higher cationic character provides improved stability. The additive according to the invention improves in addition to the wet strength of a web also the surface resistance of the product. The surface resistance is of particular significance for printing papers.

[0013] For the preparation of an additive for paper and board making according to the invention defined in claim 1, a polysaccharide having a strong cationic character is used, which in an aqueous solution is ready for use, and no time-consuming cooking is needed. The degree of substitution of the cationic polysaccharide is about 0.02-0.50. The polysaccharide used is preferably degraded starch. The starch may be any known starch, e.g. potato, corn, waxy maize, barley, wheat or tapioca starch. In addition to starch, other useful polysaccharides are e.g. dextrine, amylopectin, amylose and mannose.

[0014] In addition, vinylic monomers are used in the product according to this invention. The proportion of the monomers calculated on the dry solids content of the starting materials is not more than about 30%, preferably below 20%. Furthermore, at least about 95% of the molar amount of the monomer component is amide.

[0015] In the preparation of the product according to the invention, polymerization initiators and pH adjusting agents are used as known in the art. Suitable initiators are e.g. cerium salts, persulfates, hydroperoxides alone or together with a reducing metal salt, or azo compounds.

[0016] In the following the invention is described by means of examples.

EXAMPLE 1

[0017] Preparation of an additive for the production of paper according to the invention Cationic starch (20% aqueous solution)  860.0 g Water  55.0 g Acrylic acid (80% aqueous solution)   1.2 g Acrylamide (50% aqueous solution)  38.0 g Tert-butyl hydroperoxide   5.3 g Water  18.3 g Sodium metabisulfite   3.9 g Water  18.3 g Dry solids content 20.0% Total amount 1000.0 g

[0018] The cationic starch has a degree of substitution of 0.20; the cationization has been carried out by using a quaternary ammonium compound.

[0019] The cationic starch and water are mixed together in a reactor. The pH of the solution is adjusted to 7 by using acetic acid, and 20% of both the tert-butyl hydroperoxide and the sodium metabisulfite aqueous solutions are added. The temperature is raised to 60° C., and a nitrogen atmosphere is arranged in the reactor for the duration of the reaction.

[0020] The addition of acrylic acid and acrylamide is started, the addition lasting over a period of 120 minutes.

[0021] At the same time, the addition of the remaining amount of the solution of the tert-butyl hydroperoxide and sodium metabisulfite is started, the addition lasting over a period of 50 minutes.

[0022] After the additions, the reaction is continued for 120 minutes at 60° C. under agitation. The reaction can be initiated by various initiators, as in the above example by an oxidation-reduction-reagent pair, which includes i.a. a copper sulfate-hydrogen peroxide-pair, or e.g. by a cerium salt. The catalyst may also be a persulfate or another catalyst commonly used in emulsion polymerization.

EXAMPLE 2

[0023] Testing of Handsheets

[0024] In this test, a paper making pulp based on chemical pulp, mechanical pulp and reject from a paper mill and a laboratory sheeting mould were used. The sheets were prepared in a weight of 60 g/m². The addition of additives (cationic starch or the product according to example 1) was carried out as kg per ton dry pulp. Control point Wet tensile strength 1 2 3 4 5 kNm/kg Control 0 1.92 Cationic starch 1 1.82 Cationic starch 3 1.93 Product of Examle 1 1 2.12 Product of Example 1 3 2.13

[0025] The wet tensile strength was determined freshly on wet sheets (about 35% dry solids).

[0026] A paper strip was drawn by a measuring device at constant speed (50 mm/min) and the tensile force required to cause fracturing of the paper was registered.

[0027] The results show that the test product according to the invention improves the wet tensile strength of the sheets while the cationic starch alone does not.

EXAMPLE 3

[0028] Laboratory sheets are prepared from a pulp consisting of 80% mechanical pulp and 20% chemical pulp. The additions of the additives are given in kg/t. In the Example, the tested sheets were prepared using a fiber orientating sheeting mould in which the machine direction (MD) and the cross direction (CD) can be distinguished. Wet tensile Wet tensile Control point strength strength 1 2 3 4 MD kNm/kg CD kNm/kg Cationic starch + 1 + 1 2.06 1.35 glyoxal Cationic starch 3 2.26 1.35 Product of 1 + 1 3.32 1.67 Example 1 + glyoxal Product of 3 3.21 1.46 Example 1

[0029] It can be seen that the product according to the invention significantly improves the wet web strength both in the machine direction and in the cross direction.

EXAMPLE 4

[0030] Laboratory sheets are prepared from a pulp consisting of 15% chemical pulp, 20% reject and 55% mechanical pulp, and as filler kaolin, so that the ash level will be 20%. The additions of the additives are given in kg/t of the dry solids of the pulp.

[0031] The sheets were prepared to a basis weight of 70 g/m².

[0032] In the preparation of the sheets, also a conventional pulp starch (MT) was used which was added in the amount of 5 kg/t. Wet tensile Wet tensile IGT- Control point strength strength pick 1 2 3 4 MD kNm/kg CD kNm/kg m/s Zero point 0 2.1 1.2 2.27 Pulp starch 5 2.4 1.2 2.66 MT + 5 + 1 2.4 1.2 2.82 Product of Example 1 MT + 5 + 2 2.9 1.3 3.45 Product of Example 1

[0033] When preparing these sheets, a fiber orientating mould was used. By using the test product, the wet tensile strength was improved mainly in the machine direction when the test product was added in the amount of 2 kg/t.

[0034] Also the IGT surface resistance was measured on these sheets, and a slight improvement could also be observed compared to the zero point as well as to the use of the pulp starch alone. The IGT picking indicates how the surface of a paper, which has been printed with a printing ink, is able to resist abrasion before the surface becomes flocked and the fibers can be seen on the surface.

[0035] In addition to what is described above, a product according to the invention may be used in paper and cardboard production, for example for stabilizing hydrophobic size dispersions. Such sizes are e.g. AKD (alkyl ketene dimer) and ASA (alkenyl succinic anhydride). Thus, addition of such size dispersions to the pulp also improves the wet web strength. 

1. An additive for the production of paper and board, containing a product polymerized from at least one vinylic monomer in the presence of a cationic saccharide, characterized in that the cationic polysaccharide has a degree of substitution of 0.02-0.50, and that monomers in the amount of up to 30% calculated on the dry weight of the starting materials are used in the polymerization, whereby at least 95% of the molar amount of the monomer component is amide.
 2. The additive according to claim 1, characterized in that up to 5% of the molar amount of the monomer component is vinylic carboxylic acid.
 3. The additive according to claim 1 or 2, characterized in that monomers in the amount of up to 20% calculated on the dry weight of the starting materials are used in the polymerization.
 4. The additive according any of claims 1 to 3, characterized in that the polysaccharide is degraded starch.
 5. A use of the additive according to claim 1 in the production of paper and board.
 6. The use according to claim 5, characterized in that the additive is added in an amount of 1-3 kg per ton of the dry solids of the pulp. 